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Advanced Organic Chemistry: Mass spectrum of hexane

The mass spectrum of hexane

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing mass spectra of hexane

See also comparing infrared, mass, 1H NMR & 13C NMR spectra of the structural alkane isomers of C6H14

mass spectrum of hexane fragmentation pattern of m/z m/e ions for analysis and identification of hexane image diagram doc brown's advanced organic chemistry revision notes 

Hexane C6H14, alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b

For more see The molecular structure, classification and naming of alkanes

Interpreting the fragmentation pattern of the mass spectrum of hexane

[M]+ is the molecular ion peak (M) with an m/z of 86 corresponding to [C6H14]+, the original hexane molecule minus an electron, [CH3CH2CH2CH2CH2CH3]+.

The small M+1 peak at m/z 87, corresponds to an ionised hexane molecule with one 13C atom in it i.e. an ionised hexane molecule of formula 13C12C5H14

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of hexane.

m/z value of [fragment]+ 71 58 57 56 55
[molecular fragment]+ [CH3CH2CH2CH2CH2]+ [C4H10]+ [CH3CH2CH2CH2]+ [C4H8]+ [C4H7]+
m/z value of [fragment]+ 43 42 41 39 29 28 27
[molecular fragment]+ [CH3CH2CH2]+ [C3H6]+ [C3H5]+ [C3H3]+ [CH3CH2]+ [C2H4]+ [C2H3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of hexane

Examples of equations to explain some of the most abundant ion peaks in the mass spectrum of 2-methylpentane

Formation of m/z 71 ion:

[CH3CH2CH2CH2CH2CH3]+  ===>  [CH3CH2CH2CH2CH2]+  +  CH3

C-C bond scission in the carbon chain of the molecular ion of hexane

Formation of m/z 57 ion:

[CH3CH2CH2CH2CH2CH3]+  ===>  [CH3CH2CH2CH2]+  +  CH2CH3

The m/z 57 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Again, C-C bond scission in the carbon chain of the molecular ion of hexane.

Similarly the m/z 57 ion could be formed by further scission of a shorter ion

e.g.: [CH3CH2CH2CH2CH2]+  ===>  [CH3CH2CH2CH2]+  +  CH2

Formation of m/z 43 ion:

[CH3CH2CH2CH2CH2CH3]+  ===>  [CH3CH2CH2]+  +  CH2CH2CH3

Formation of m/z 29 ion:

[CH3CH2CH2CH2CH2CH3]+  ===>  [CH3CH2]+  +  CH2CH2CH2CH3

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the five structural alkane isomers of C6H14

NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane image sizes.  These five molecules are structural isomers of saturated alkanes of molecular formula C6H14 and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic alkanes (non-cyclic alkanes).

Infrared spectra below.

INFRARED SPECTRA:

Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, there are no other great striking differences, but each could be identified from its infrared spectrum.

All the absorption bands are typical of molecules containing saturated alkyl structure and there are no characteristic infrared absorptions due to a specific functional group.

Infrared spectra above, mass spectra below.

MASS SPECTRA: Base ion peaks plus m/z comments.

Hexane: m/z 57, 42 and 56 prominent

2-methylpentane: m/z 43, 42 and 71 prominent

3-methylpentane: m/z 57, 41 and 56 prominent

2,2-dimethylbutane: m/z 43, 41, 57 and 71 prominent

2,3-dimethylbutane: m/z 43, 41, 42 and 71 prominent

Mass spectra above, 1H NMR spectra below.

1H NMR SPECTRA: They can all be distinguished by their different integrated proton ratios - need very high resolution.

Hexane: 3 1H δ shifts, H ratio 3:2:2 (6:4:4 in formula)

2-methylpentane: 5 1H δ shifts, H ratio 6:3:2:2:1

3-methylpentane: 4 1H δ shifts, H ratio 6:4:3:1

2,2-dimethylbutane: 3 1H δ shifts, H ratio 9:3:2

2,3-dimethylbutane: 2 1H δ shifts, H ratio 6:1 (12:2 in formula)

1H NMR spectra above, 13C NMR spectra below.

13C NMR SPECTRA: From the number of shifts, you can't distinguish (iii) and (iv) but you can distinguish them from (i), (ii) and (v). (i) Hexane: 3 13C δ shifts

(ii) 2-methylpentane: 5 13C δ shifts

(iii) 3-methylpentane: 4 13C δ shifts

(iv) 2,2-dimethylbutane: 4 13C δ shifts

(v) 2,3-dimethylbutane: 2 13C δ shifts

13C NMR spectra above.

Key words & phrases: image diagram on how to interpret and explain the mass spectrum of hexane m/z m/e base peaks, image and diagram of the mass spectrum of hexane, details of the mass spectroscopy of hexane,  low and high resolution mass spectrum of hexane, prominent m/z peaks in the mass spectrum of hexane, comparative mass spectra of hexane, the molecular ion peak in the mass spectrum of hexane, analysing and understanding the fragmentation pattern of the mass spectrum of hexane, characteristic pattern of peaks in the mass spectrum of hexane, relative abundance of mass ion peaks in the mass spectrum of hexane, revising the mass spectrum of hexane, revision of mass spectroscopy of hexane, most abundant ions in the mass spectrum of hexane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of hexane, how to analyse the mass spectrum of hexane, how to describe explain the formation of fragmented ions in the mass spectra of hexane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of hexane recognising the base ion peak of hexane interpreting interpretation the mass spectrum of hexane

Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of hexane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of hexane. The m/e m/z value of the molecular ion peak in the mass spectrum of hexane.  The m/e m/z value of the base ion peak in the mass spectrum of hexane. Possible examples of equations showing the formation of the ionised fragments in hexane. Revision notes on the mass spectrum of hexane. Matching and deducing the structure of the hexane molecule from its mass spectrum. Mass spectroscopy of  aliphatic alkanes, mass spectra of hexane, an isomer of molecular formula C6H14


Links associated with hexane

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of hexane

The H-1 NMR spectrum of hexane

The C-13 NMR spectrum of hexane

Mass spectroscopy index

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